Dentinal tubule

A dentinal tubule is a microscopic channel in dentine, within the mineralized material formed by odontoblasts. The tubule network records the paths of odontoblast processes as dentine is deposited. A tubule is a structural channel; whether a prepared specimen contains fluid, a cellular process or an empty space depends on the tissue and its preparation.^[ref]

Elephant ivory provides a useful structural example. In a2017 study, observations of different cut planes and computed tomography using synchrotron radiation connected visible tubule profiles with their three-dimensional trajectories. A tube cut across, along or obliquely does not present the same two-dimensional profile. Consequently, a collection of oval profiles in a section need not consist of isolated oval cavities.^[ref]

A deliberately simplified transport example makes the channel size matter. For steady laminar flow through a straight circular tube, the Hagen-Poiseuille equation gives $Q=\pi r^4\Delta p/(8\mu L)$, where $r$ is tube radius, $L$ length and $\mu$ fluid dynamic viscosity. Doubling $r$ would multiply $Q$ by16 at fixed pressure difference and length. Real dentinal tubules branch, vary in width and may be obstructed, so this ideal calculation illustrates sensitivity to geometry rather than predicting a whole tooth's permeability.